Could you brief us on some of your projects which have used fly ash?
Afcons has completed over a dozen projects in India and abroad by replacing 20-35 per cent of cement with fly ash. We started with Tata coal berth project in 2005, where we used 35 per cent of fly ash. In 2006, we used fly ash for the construction of a flyover project in Pune in M75 Grade High performance concrete. In 2007, in MIHAN project near Nagpur, we used fly ash up to 20 per cent.In Kol dam project in 2008, we used 30 per cent fly ash; and in 2010 in Pipavav Drydock, we used 35 per cent fly ash. In Chennai metro project, we are using 28 per cent of fly ash and we are also using fly ash in the Kolkata metro rail project.
What should be the preferred practice of using fly ash?
The decision on using fly ash in concrete should come in the initial stage of the project contract. One has to understand why fly ash is being added to the concrete; what are the properties of fly ash; what kind of fly ash is available in the nearby area; and this needs to be tested by a recognized laboratory. Based on the results, we need to design and develop the concrete mix. We have to analyse the results and accordingly decide on the amount/percentage of fly ash to be added into the concrete. Simultaneously, there are many tests we have to carry out for the fresh and hardened concrete, and come to a conclusion on the use of fly ash.
What are the economic benefits of using fly ash in concrete?
This depends on how far is the thermal power plant from the project site. If it is far away, then the transportation cost will go up. Most of the time, it will come to the same cost of the cement or little bit lower. Normally, even though the cost of fly ash is lower compared to cement, other factors like separate silos for storage of fly ash, a stringent quality control system, etc add to the cost. Proper attention of concrete placing, compaction and curing is needed. Proper curing will ensure presence of moisture which is necessary for the secondary reaction in fly ash based concrete.
How do you procure fly ash for your projects?
We will make available the fly ash depending on the progress of the project. We keep stock of certain quantity of fly ash in advance so that even if there is a delay in getting the fly ash due to certain issues, the project will not stop. Normally, we source fly ash through suppliers. Currently we get fly ash through two suppliers, Ashtech India and Dirk. We select the nearby plants to source fly ash and do testing to match with our requirement as per IS 3812.
What are the key factors to be taken care of while using fly ash in concrete?
The fly ash has to be tested before using. If the requirements are as per IS 3812, ensure that all these parameters are met with then only you can proceed with mix design, take trials and see the results. Fresh and hardened concrete property requirements are important aspects to be taken care of. Proper storage of fly ash is of utmost importance and fly ash should be stored like cement. If fly ash is stored for a longer time, then before using it, you need to retest to ensure the parameters stipulated are met. Another factor is the lime reactivity. the percentage of strength achieved in comparison with OPC mix need to be checked.
What is the awareness level of using fly ash in concrete?
The awareness is gradually increasing; however, people are stuck because many contracts do not specify the use of fly ash properly in the tender document. If the owner is specifying it clearly in the contract document, it will be used in the concrete as a cement replacement, to the percentage he wants. There are two types of fly ash concrete - normal fly concrete and high volume fly ash concrete. In high volume fly ash concrete normally more than 30 per cent fly ash is used. To design and develop fly ash based concrete mixes proper R&D has to be carried out. This will be project-specific and can vary from project to project. One has to carry R&D at his project and come out with proper concrete mix design, and then go ahead with it. At Afcons, we have laboratories at every project sites for testing and R&D. Once the project is over, then we shift to another project.
What is the scope of using fly ash in concrete? What are Afcon´s initiatives towards this?
Certain government organisations have amended the norms for contracts making the use of fly ash mandatory in concrete. The use of fly ash by partially replacing cement in concrete mix helps in minimising carbon emission which happens in manufacturing cement. At Afcons, one of our objectives is to use fly ash or ground granulated blast furnace slag (GGBS), a byproduct of steel industry, in our projects.
When we take up a new project, if the contract does not mention about the use of fly ash, we go to the client to make them understand the benefits of using fly ash and how this will improve the quality of their structure. We make them aware of the benefit of using fly ash or slag in terms of the durability of the structure. We have also worked with a number of government projects where we have used fly ash.
What additional benefits the use fly ash brings to concrete?
The addition of fly ash, a byproduct of thermal power plants into concrete has many advantages especially in terms of durability and strength of concrete. In normal concrete mix containing only OPC, when water and cement comes in to contact, chemical reaction takes place producing binding material and consolidates the concrete mass. The process is exothermic and heat is released which increases the temperature of the mass causing concrete to crack. Fly ash is very light fine particles with cementitious properties. When fly ash is present in the concrete mass, it plays dual role - fly ash reacts with the free lime in concrete mix and produces binder and renders additional strength to the concrete mass and making it denser.
When water is added into concrete, chemical reaction (hydration) starts between water and cement. In this reaction, the unreacted calcium hydroxide creates capillaries in the concrete and destroys the durability of the concrete. It becomes more permeable and external particles attack the concrete through this path. So, in order to enable a secondary reaction for calcium hydroxide, we add fly ash. When fly ash is added into concrete, the silica in the fly ash takes part in the secondary reaction with the free calcium hydroxide in the concrete and forms C-S-H gel. After this reaction, there will be minimum or very little free calcium hydroxide remain unreacted. Thus the concrete becomes more durable.